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Chemistry LibreTexts

2.6 Batteries

  • Page ID
    32278
  • Electrochemical cells used for power generation are called batteries. Although batteries come in many different shapes and sizes there are a few basic types. You won't be required to remember details of the batteries, but some general information and features of each type is presented here.

    1. Primary batteries - (dry cell batteries)

    • non-rechargeable

    • electrolytes are present as a paste rather than as a liquid
    • general purpose battery used for flashlights, transistor radios, toys, etc.
    • The basic dry cell battery consists of: zinc case as the anode (oxidation); a graphite rod is the cathode (reduction) surrounded by a moist past of either MnO2, NH4Cl, and ZnCl2 or in alkaline dry cells a KOH electrolytic paste.
    • General reactions for the battery - manganese(IV) oxide-zinc cell (different batteries have different reactions - you don't need to remember any of these reactions)
    cathode

    \(\ce{2MnO2(s) + 2NH4+ + 2e- -> Mn2O3(s) + H2O(l) + 2NH3(aq)}\)

    anode

    \(\ce{Zn(s) -> Zn^{2+}(aq) + 2e-}\)

    • Maximum voltage 1.5V. By connecting several cells in series 90V can be achieved.
    • Advantages of alkaline batteries - consistent voltage, increased capacity, longer shelf-life, and reliable operation at temperatures as low as -40°C
    • Disadvantage - higher cost

    2. Secondary Batteries (storage batteries)

    • rechargeable
    • an example - lead-acid battery used in cars. Anode is grid of lead-antimony or lead-calcium alloy packed with spongy lead; Cathode is lead(IV) oxide. Electrolyte is aqueous sulfuric acid. Consists of numerous small cells connected in parallel (anode to anode; cathode to cathode).
    • General reaction:
    cathode

    \(\ce{PbO2(s) + 4H+ + SO4^{2-}(aq) + 2e- -> PbSO4(s) + 2H2O(l) + 2NH3(aq)}\)

    anode

    \(\ce{Pb(s) + SO4^{2-}(aq) -> PbSO4(s) + 2e-}\)

    • Secondary batteries are recharged by passing a current through the battery in the opposite direction. In a car battery this occurs when the engine is running.
    • Other examples include the nickel-iron alkaline battery, nickel-zinc batter, nickel-cadmium alkaline battery, silver-zinc, silver-cadmium

    3. Fuel Cells

    • fuel cells are electrochemical cells that convert energy of a redox combustion reaction directly into electrical energy. Requires a continuous supply of reactants and a constant removal of products.
    • Cathode reactant usually air or pure oxygen; anode fuel is a gas such as hydrogen, methane, or propane. Carbon electrodes typically contain a catalyst. The electrolyte is typically KOH.
    • General reaction:
    cathode

    \(\ce{O2(g) + 2H2O(l) + 4e- -> 4OH-(aq)}\)

    anode

    \(\ce{2H2(g) + 4OH-(aq) -> 4H2O(l) + 4e-}\)

    net

    \(\ce{2H2(g) + O2(g) -> 2H2O(l)}\)

    • Advantages - no toxic waste products (water is the only product); very efficient energy conversion (70-80% efficient)
    • Disadvantage - too expensive for large-scale use.